With an explosive spread of data communication networks such as the Internet, optical communication networks having a larger capacity are increasingly required. In order to satisfy the demand for such networks, multiple-wavelength communication has been put into a practical use. In addition, another demand has been found in recent years for a wavelength-selective switch (WSS: Wavelength Selective Switch) enabling forwarding switching. One of conventional wavelength-selective switch is disclosed in Non-Patent Literature 1.
FIG. 12 illustrates a wavelength-selective switch disclosed in Non-Patent Literature 1. The wavelength-selective switch described in Non-Patent Literature 1 discloses that a WSS input optical system includes a lens optical system configured on a waveguide that is used to simplify the WSS optical system to thereby realize a smaller size and a lower price. Specifically, an optical waveguide formed on a substrate 100 is integrated with the WSS input optical system. An optical signal inputted through an input waveguide 101 enters an array waveguide 103 via an input slab waveguide 102. The array waveguide 103 is formed to have the same length as that of each waveguide. Gaussian beam is outputted from an output end Point A the output slab waveguide 104 to have a wide width.
When the input slab waveguide 102-side waveguides in the array waveguide 103 have a pitch d1 equal to a pitch d2 between the output slab waveguide 104-side waveguides (i.e., when d1=d2 is established), then the Gaussian beam at the above-described output end Point A has a width W that is represented by the following equation (1) when assuming that the input waveguide 101 in a waveguide mode has a width w0.
                    [                  Equation          ⁢                                          ⁢          1                ]                                                            W        =                              w            0                    ⁢                                    1              +                                                (                                                            λ                      ⁢                                                                                          ⁢                                              f                        slab                                                                                    π                      ⁢                                                                                          ⁢                                              n                        s                                            ⁢                                              w                        0                        2                                                                              )                                2                                                                        (        1        )            
A collimated beam having the width W shown in the equation (1) is also outputted from the waveguide output end. Furthermore, an optical signal inputted from an input waveguide other than the input waveguide 101 (i.e., from the input waveguide 101b) is outputted from the waveguide output end as a beam whose beam center exists in the above-described output end Point A and whose inclination corresponds to the distance x between the input waveguides 101 and 101b. 
The input optical system formed on the substrate 100 shown in FIG. 12 is suitable for a switch configuration in which the space optical system has 4f. As shown in FIG. 12, 4f is shown by an equation in which 4f=(distance f from waveguide chip 100 to lens 105a)+(distance f from lens 105a to the diffraction grating 106)+(distance f from the diffraction grating 106 to lens 105b)+(distance f from lens 105b to LCOS 107).
The above-described space optical system is suitable for an optical system in which the same optical power element (lens) is used for both of the switch axial direction and the wavelength direction to perform a wave front formation. In FIG. 12, an optical signal outputted from the substrate 100 enters, via two lenses 105a and 105b and a diffraction grating 106, the LCOS 107 functioning as a light deflection element. However, an optical signal inputted through a different input port (e.g., input ports 101 and 101b) enters the LCOS 107 at a different angle. The principal rays of these beams reach the LCOS 107 at the same position. Thus, the LCOS 107 can be used to deflect the beam to optically-couple the optical signals from the different input ports, thereby realizing a switch function.
The optical system shown in FIG. 12 is suitable for the optical system of 4f but cannot be applied to optical systems having the other configurations. For example, in the case of the 2f optical system as shown in FIG. 13, an optical signal outputted from the input fiber 201a is sent through a light path 204 and a lens 202 and enters the LCOS 203 functioning as a light deflection element. Then, the incident light is deflected by the LCOS 203 and is coupled by optical fibers 201b via the light path 205. In this case, the optical signal preferably enters the LCOS 203 as a collimated beam. The optical signal from the input fiber 201a propagates as a diffusion beam 204a in a range in front of the lens 202 (a range between the input fiber 201a and the lens 202).
In the case of the 2f optical system, a conventional input optical system made on a substrate cannot be used, thus failing to provide a simple optical system or a simple assembly. In the 2f optical system, the optical fibers 201a and 201b preferably output beams for which the principal rays thereof are parallel to each other and the diameters are relatively small. However, a conventional configuration is not suitable for the formation of such a beam. Ideally, the output beam from the optical fibers is preferably small enough to form a Fraunhofer region in order to propagate by a lens focal point distance f. When the output beam is too small on the other hand, the beam expands excessively while propagating the space. Thus, as shown in FIG. 14, a measure is also required according to which the output ends of the optical fibers are provided with micro lens arrays 205, respectively, to provide a beam waist having a certain size and to suppress the beam waist sizes of the optical fiber output end, thereby reducing the numerical aperture.
The necessity as described above is more increased in the case of the optical system disclosed in Patent Literature 1. FIG. 15 is the same as FIG. 7 of Patent Literature 1 and shows the configuration of a wavelength selective switch in which an LCOS is used as a switching element. In the wavelength selective switch shown in FIG. 15, the LOCS element has a polarization dependency and thus requires a polarization diversity optical system in which the polarization status of input light to the LCOS is aligned in one direction. In the polarization diversity optical system as described above, a polarization separation element 215 is used to separate optical signals from the respective input fibers 201 to 205 (see beams 291 and 294) to beams of orthogonal polarization components (see beams 292 to 296). Thereafter, an optical element in which glass 221 and λ/2 wave plates 222 are arranged in a ladder-like manner is used to align the separated beams of the orthogonal wavelength components. In this case, the optical element 220 must have a very-small opening consisting of the glass 221 and the λ/2 wave plates 222 and the interior must allow incident light to pass through without being blocked.
In FIG. 15, the reference numeral 210 denotes a micro lens array and the reference numeral 230 denotes a birefringent wedge element.